In fabrication of CMOS devices for VLSI integrated circuits, tungsten is a preferred alternative to conventional aluminium (Al) alloys for metal interconnect and for submicron contacts and vias, particularly where high current density is required. As device dimensions become smaller and the packing density of high speed devices increases, reduced metallization linewidth requires the formation of smaller contacts and vias with vertical sidewalls. In use of tungsten for contacts and vias, tungsten has high resistance to electromigration, provides superior step coverage and does not form hillocks. On the other hand, thin layers of tungsten have a higher sheet resistance than conventional aluminium alloys. For example, for use of tungsten as interconnect metallization, deposition of a film of about 1 .mu.m thickness of tungsten may be required to provide the desired resistivity, of 8-9 .mu..OMEGA.cm, typically .about.8.3 .mu..OMEGA.cm.
A thin film of tungsten may be formed by chemical vapour deposition (CVD). In a conventional CVD process, tungsten hexafluoride, WF.sub.6, is reduced by hydrogen, H.sub.2, in argon carrier gas, at a pressure of .about.1 Torr. The superior step coverage of CVD tungsten film, relative to sputtered aluminium alloys, provides that vias and contact holes can be filled by blanket deposition of tungsten overall and subsequent etching back of excess tungsten to leave tungsten only in contact holes and vias. In this scheme, aluminium is used as metal interconnect. In use of tungsten for interconnect applications, a conformal CVD tungsten film may be deposited which is capable of filling holes having high aspect ratios and providing contact plugs in vertical walled contact holes.
Tungsten does not adhere well to common dielectric materials, including silicon dioxide SiO.sub.2. Satisfactory adhesion to the substrate is required for tungsten films both for filling of contact and via openings and for use of tungsten as interconnect. A known method of improving the adhesion of tungsten to substrate is deposition of an adhesion layer, for example TiN or TiW, before deposition of tungsten. The adhesion layer allows for formation of strong chemical bonds between layers of tungsten and a dielectric such as SiO.sub.2.
Known processes for deposition of films of tungsten metallization of 1 .mu.m thickness and having the desired resistivity produce films with a rough surface and high film stress. During deposition of thin tungsten films (&lt;2000 .ANG.) using a conventional CVD process, in which WF.sub.6 is reduced by H.sub.2 in argon carrier gas, at a pressure of .about.1 Torr, the tungsten film initially has small grains, but the grains grow and form long columnar grains in the growth direction as film thickness increases. The columnar grains have pyramidal caps forming the surface of the film. Consequently, the resulting film has a rough surface. Also, film of this structure has a tensile film stress of .about.10.sup.10 dyne/cm.sup.2. This stress is much greater than that desirable to match, or compensate, the film stress of an underlying film of dielectric, such as SiO.sub.2, which typically has a compressive film stress of .about.2 .times.10.sup.9 dynes/cm.sub.2.
Surface roughness of tungsten films deposited by known CVD processes increases with increasing film thickness. Surface roughness affects the optical properties of the film and reduces specular reflectivity (and conversely increases diffuse reflectivity) in the spectral region used for photo-lithography (around 436 nm). The recognition of alignment marks may be repeatable and accurate only for a film thickness less than 5000 .ANG.. Typical tungsten film of .about.8000 .ANG. thickness, deposited by known CVD processes which provides good step coverage, has a specular reflectivity of 20% or less compared to a silicon reference wafer. When the specular reflectivity of a rough film is reduced to such an extent, or if surface roughness is significant enough to obscure alignment marks, subsequent photo-lithographic alignment for patterning of interconnect structure is impossible. Consequently use of thick tungsten films for interconnects is impracticable.
Another disadvantage of conventional processes for deposition of a thin film of tungsten on silicon by reduction of WF.sub.6 with hydrogen results from a significant amount of volatile products such as HF, and the presence of WF.sub.x species, which attack the underlying silicon, and result in undesirable etching and pitting of the silicon surface during deposition.
In another known method of chemical vapour deposition of tungsten by reduction of WF.sub.6 with silane it is found that there is reduced pitting and etching of a silicon substrate. A higher rate of deposition may be obtained by reduction of WF.sub.6 with silane instead of hydrogen. The resulting tungsten film has a smoother surface. However, the resulting film has poor step coverage, and adhesion to the underlying silicon is inferior. Poor adhesion may result in problems such as lifting of the film and particulate contamination, as well as poor electrical performance and reliability.
To improve adhesion, U.S. Pat. 5,028,565 to Chang et al. for example, describes use of an adhesion layer, followed by deposition of a thin nucleation layer of tungsten before deposition of the major thickness of tungsten by a method of CVD of tungsten using a conventional mixture of reactive gases including WF.sub.6, H.sub.2, in a carrier gas of argon, carried out in the presence of nitrogen. The '565 patent discloses that deposition in the presence of nitrogen and at higher pressures up to 760 Torr, preferably at .about.80 Torr, together with high gas flow rates, and elevated temperature, 450.degree.-475.degree. C., resulted in smoother tungsten films. The resulting tungsten film showed increased specular reflectivity of the deposited tungsten surface, near 100% relative to silicon, which facilitated the use of photolithography in the subsequent patterning step. However, when the tungsten deposition was carried out in the presence of nitrogen at low pressure, at 10 Torr, the specular reflectivity of the tungsten film was reduced to only 20%. The increased pressure, together with high gas flow rates and increased temperature, resulted not only in a tungsten film having a smoother surface, but also in increased tungsten deposition rates by up to an order of magnitude (2000-7000 .ANG./min) compared with conventional known methods (.about.300 .ANG./min).
On the other hand, as a practical matter, it is not possible to operate many known CVD reactors at the higher pressures, .gtoreq.80 Torr, required for the method disclosed in the '565 Patent to Chang to achieve deposition of tungsten films having a smooth surface suitable for interconnect. Thus, practical difficulties are encountered in using low pressure CVD to provide tungsten films having good adhesion, a smooth surface for use as interconnect and satisfactory step coverage for filling contact via holes.